The duplex model: Implications from a study of flexural-slip duplexes

Abstract

Criteria for the recognition of duplexes developed on the limbs of flexural-slip folds are summarized and the morphology of these flexural-slip duplexes is compared with that of well-documented, fully-exposed examples of duplexes from thrust belts. The two groups of structures have many features in common regardless of scale or of the rock type in which they occur. They exhibit flat or gently curved floor and roof thrusts; low-angle thrusts in both the tip and rear of the structure; and limited rotation of horses in the middle portion. In marked contrast to current geometrical models for duplex formation, the development of ramp anticlines above newly-formed thrusts at the tip of the duplex is relatively unimportant in natural examples. A revised model for duplex formation is proposed in which new horses are accreted to the tip of the wedge-like structure on low-angle thrusts, are then rotated between active floor and roof thrusts as the structure grows, and become sigmoidal in the interior part of the duplex. The duplex maintains its streamlined profile by internal adjustments, some oblique to the main movement direction, on both link thrusts and backthrusts; all movements are accompanied by the growth of quartz or calcite fibre veins on thrust surfaces.